A method of preparing a californium-252 neutron

ABSTRACT

A method of encapsulating a radioisotope wherein an open ended tube is used first to support a porous member for filtering a radioisotope precipitate from a solution and next for containing the precipitate on the porous member while heating to convert the radioisotope to a thermally stable form. The tube is then sealed at both ends providing a leaktight capsule to prevent loss of the radioisotope. This method is particularly applicable to the encapsulation of californium-252 for use as a point source of neutrons.

United States Patem inventors Alexander R. Boulogne;

Jean P. Faraci, both of Aiken, S.C.

Appl. No. 1,484

Filed Jan. 8, 1970 Patented Dec. 14, 1971 Assignee The United States ofAmerica as represented by the United States Atomic Energy Commission 5 6References Cited UNITED STATES PATENTS 2,830,190 4/1958 Karp 252/301.1 X3,l24,538 3/1964 Lewis 252/301.1 2,592,1l5 4/1952 Carroll 252/30].] X

Primary Examiner-Carl D. Quarforth Assistant Examiner-R. L. Tate IAnomey- Roland A. Anderson METHOD OF PREPARING A CALlFORNlUM-252 NEUTRON4 Claims, 2 Drawing Figs.

US. Cl 252/301.151, 250/l06 S, 264/05, 23/343 Int. Cl C09k 3/00, G2 1 h5/00 Field of Search 252/301.1;

is particularly applicable to the encapsulation of californium- 252 foruse as a point source of neutrons.

as as VACUUM SOURCE FATENTEU DEC] 4197i VACUUM SOURCE lN VENTOR.Alexander R Boa/ogne By Jean P. Farac/ Afro/nay:

BACKGROUND OF THE INVENTION The present invention was made in the courseof or under a contract with the U.S. Atomic Energy Commission.

1. Field of the Invention The present invention relates to theencapsulation of various radioisotopes in varying quantities but moreparticularly to the encapsulation of small quantities of rare andexpensive radioisotopes to form point sources of radiation. For purposesof this application a point source shall be defined as one containing asmall concentrated mass of radioisotope such that radiation emanatingtherefrom generally appears to originate at a point.

The spontaneous fission of only microgram quantities of *Cf provides asignificant flux of neutron radiation. Consequently, an encapsulatedsmall mass of this element has great value as a portable neutron sourceand may be employed in areas such as radiography, neutron activationanalysis of unknown materials and mineral exploration. Point sources areespecially useful for producing sharp images on neutron radiographs.

2. Description of Prior Art Prior califomium point sources have beenprepared by coprecipitating -""-Cf(I'I with I- e(0I-I) from a purifiedsolution. (International Journal of Applied Radiation and Isotopes I969,Vol. 20, pp. 45346l, Pergamon Press.) In this prior process, theresulting gelatinous precipitate is centrifuged from the remainder ofthe solution and drawn into a pipette for transfer to a small platinumcone. The precipitate is dehydrated, the cone is mechanically folded toform a pellet and the Cf(0I-i and Fe(0I-I inside the pellet are calcinedto form oxides. The pellet is then suitably encapsulated forutilization.

This prior process has several limitations and disadvantages.Radioisotopes, especially califomium, with which this prior process isused are rare and costly elements. Unavoidable losses can occur as thegelatinous precipitate is transferred by pipette or as the conecontaining the radioisotope is folded into a pellet. Precision in thequantity of radioisotope in the source is difficult to achieve becauseof these losses and because of the uncertainty in the amount ofradioisotope in the gelatinous precipitate drawn into the transferpipette. Moreover, the transfer and mechanical folding steps arecumbersome to perform with remote handling equipment within a shieldedcompartment.

SUMMARY OF THE INVENTION In view of the limitations of the prior art itis an object of the present invention to provide an economical method ofencapsulating a precise amount of a radioisotope.

It is also an object to provide a convenient method for remotelypreparing a radiation source within the confines of a shieldedcompartment.

It is a further object to provide a method of preparing a neutron sourcecontaining califomium isotopes.

In accordance with the present invention a method is provided forencapsulating a radioisotope to be used as a radiation source. An openended tube or capsule is used to support a porous member for filtering acrystalline precipitate includ ing the radioisotope from solution. Theprecipitate is calcined within the capsule to a more thermally stableform. The capsule is then sealed at both ends to complete theencapsulation of the radioisotope.

The limitations of the prior art are practically eliminated by thesequential use of the source capsule for the steps of filtration,calcination and encapsulation of the radioisotope.

The present method of encapsulation is particularly applicable to thepreparation of point radiation sources of rare and expensiveradioisotopes, such as isotopes of califomium.

DESCRIPTION OF THE DRAWINGS The present invention is illustrated in thefollowing drawings wherein:

FIG. 1 is a simplified elevation view partially in cross section of oneapparatus for precipitating and filtering a radioisotope.

FIG. 2 is a cross-sectional view of an encapsulated radioisotope.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, anassembled apparatus is shown for implementing the method of the presentinvention. A precipitator vessel 11 containing a slurry 13 ofradioactive precipitate 37 and solution is provided with an opening 15for admitting the process ingredients. A threaded outlet 21 permitsslurry 13 to flow from the precipitator vessel 1 1.

A first transition member 19 is sealingly attached to precipitatorvessel 11 at outlet 21. A tapered projection 27 is formed in transitionmember 19 at the end opposite the precipitator vessel 11. A passageway23 extends from outlet 21 of precipitator vessel 11 through the taperedprojection 27 of transition member 19.

A radiation source capsule 29, in the shape of an open ended tube, has alongitudinal passageway 31 with tapered openings 33 at both ends.Projection 27 of transition member 19 is tapered to correspond to andsealingly fit into mating relationship with an opening 33. A porousmetal member 35 is fitted against a chamfer 36 in passageway 31 to serveas a filter for radioactive precipitate 37. Porous member 35 can be apressed and sintered compact of particulate stainless steel designed toretain particles greater than about 10-20 microns in diameter.

A second transition member 39 connects the source capsule 29 with afiltrate receptacle 41. The second transition member 39 is similar totransition member 19 except that an extension tube or downspout 43extends into receptacle 41 to below the level of a port or connection 47provided for engaging a vacuum source. A passageway 45 is providedthrough transition member 39 and downspout 43 to allow filtrate 49passing through porous metal member 35 to enter receptacle 41.

The assembled apparatus shown in FIG. 1 is normally disposed in ashielded compartment provided with ordinary remote handling devices orslaves" to begin preparation of a radiation source. The assembly isinitially arranged horizontally with opening 15 of precipitator vessel11 in an upward position.

A solution containing a measured amount of the desired radioisotopes,such as Ct, and possibly Ct, is transferred to the precipitator vessel11. A second ingredient, such as oxalic acid (11 C 00, either insolution or pure form, is added in substantial stoichiometric excess tovessel 11 to precipitate the radioisotope as a crystalline compound,such as califomium oxalate [Cf,(C,0 The hydrogen ion concentration insolution should be limited to allow the H C O to be in equilibrium witha substantial quantity of C 0? for reacting with califomium. A carrierelement, such as terbium, can also be added to the solution to ensureprecipitation of substantially all of the radioisotope. Terbium ischemically compatible with califomium and does not form undesirableradioisotopes on neutron bombardment.

The assembly is then rotated to a vertical position as shown in FIG. Ito allow slurry 13 with the crystalline precipitate 37 to flow intocontact with porous metal member 35. A suitable clamping device (notshown) is provided to support the assembly. As an alternative theassembly may be maintained in the vertical orientation shown with avalve between vessel 11 and capsule 29 to control the flow of slurry. Avacuum source is coupled to port 47 of the filtrate receptacle 41 todraw the filtrate solution 49 into receptacle 41. Crystallineprecipitate 37 is thereby deposited on porous member 35.

FIG. 2 shows a fully encapsulated radiation source which is completed asfollows. Capsule 29 is removed from the abovedescribed assembly of FIG.1! and placed in a suitably sized pressing device. A second porous metalmember is pressed into longitudinal passageway 311 in capsule 29 to alocation just above the precipitate. A compartment 52 for containing theradioisotope is thereby defined. This second porous member 51, althoughnot necessary for operability, is preferably included to retain theradioisotope while allowing volatiles to escape in the following heatingstep.

Capsule 29 with precipitate 37 is then heated to a sufficienttemperature to convert the precipitate to a thermally stable form of theradioisotope. Gaseous products, for instance carbon dioxide and steam,pass through the porous members 35 and 51 and leave a dried and calcinedproduct 53 such as califomium oxide cr o within compartment 52.

impermeable end plugs 55 are next pressed and welded into the taperedopenings 33 of capsule 29. For safety and handling convenience, capsule29 is placed in an outer housing or outer capsule 57 and sealed thereinwith an end closure 59 welded or otherwise permanently fixed in place.Outer capsule 57 is shown provided with a threaded end portion 6H forattachment to a utilization means.

EXAMPLE I It was desired that a point source of neutron radiation beprepared containing 700 micrograms of Cf. Three milliliters of 1M HNOcontaining 730.6 micrograms of "*Cf, 4 ml. of distilled water and 0.5ml. of 1M H C O were combined in a precipitator vessel. A whitecrystalline precipitate of Cf (C O., began to form after about l2minutes. After about minutes of reaction time the crystallineprecipitate was filtered on a porous stainless steel disk which waspreviously pressed into a platinum-rhodium source capsule. Ninety-twopercent by weight of the Cf introduced into the precipitator vessel wascaptured in the filtered precipitate. The filtrate, containing theremainder of the Cf, was further treated with 500 micrograms of terbiumnitrate [Tb(NO dissolved in 0.25 ml. 1M l-lNC) and 0.5 ml. of 1M H C OPrecipitate including both Tb (C- ,O and Cf,(C O was allowed to form forabout 50 minutes before again filtering through the porous stainlesssteel disk in the source capsule. Ninety-six and two-tenths percent ofthe Cf in the original feed solution was recovered after this secondfiltration. The source contained 702.6 micrograms of Cf which was within0.4 percent of the desired quantity (700 micrograms). The filtratecontaining the remainder of the cf was retained for further processing.A second porous stainless steel disk was pressed into theplatinum-rhodium capsule above the oxalate precipitate. The capsule wasthen air dried at 200 C. in a quartz tube oven for about 30 minutes. Thetemperature was then slowly raised to 550 C. for about another 30minutes to complete the conversion of Cf, ,(C O,,) to Cf 0Platinum-rhodium end plugs were inert gas welded by thermal fusion intothe openings at both ends of the capsule. After testing for leaks anddecontaminating, the platinum-rhodium capsule was sealed inside a 304-Lstainless steel outer capsule.

EXAMPLE ll A second encapsulated Cf neutron source was prepared insubstantial accordance with the method employed in exampie 1. Theprecipitator vessel was loaded with 49.86 micrograms of o: in 3 ml. 0.1MHNO and with 500 micrograms micrograms terbium in nitrate solution.After about 15-20 minutes the precipitate had formed and was processedas in Example I. 97.6 percent by weigh or 48.6 micrograms of the (If inthe feed was deposited within the capsule. The lower original nitricacid concentration used in this example than used in example I providedmore available oxalate ions for reaction and consequently a shorterreaction time was required.

Although the foregoing examples only describe the encapsulation ofcalifomium to form point sources, other radioisotopes such as curium,americium and plutonium isotopes may be encapsulated by the method ofthe present invention. Furthermore, radiation sources having amounts ofradioisotopes than can be contained in point sources may be prepared.

A method is described herein for preparing a source of radiation. Themethod provides a high yield of expensive radioisotopes from the feed tothe finished source. Precise quantities of the radioisotope aredeposited into the source capsule to allow sources of exactly thedesired intensity to be prepared. lFew mechanical steps are required tofacilitate remote performance of this method within a shieldedcompartment.

What is claimed is:

l. A method of encapsulating a califomium-252 radioisotope for use as aneutron source which comprises:

a. placing a source capsule tube, having a first porous metal memberwithin one end of the passageway thereof, in mating relationship with anoutlet of a precipitator vessel;

b. precipitating a slurry of crystalline califomium oxalate in saidprecipitator vessel;

c. filtering said slurry through said first porous member to depositsaid califomium oxalate precipitate on said first porous member;

d. disposing a second porous member in the opposite end of said sourcecapsule tube passageway from said first porous member to enclose saidprecipitate between said porous members;

e. calcining said califomium oxalate deposited on said first porousmember in said capsulate tube to a thermally stable califomium compoundwhile simultaneously discharging gaseous products through said porousmembers; and

f. sealing said capsule tube with impervious end plugs over said porousmembers.

2. The method according to claim 1 wherein said capsule tube andimpervious end plugs are a platinum-rhodium alloy and said porousmembers are pressed and sintered particulate stainless steel forretaining precipitate particles having dimensions in excess of about10-20 microns.

3. The method according to claim 1 wherein said slurry additionallycontains terbium oxalate.

4. The method according to claim I wherein said californiurn oxalate iscalcined to califomium oxide.

2. The method according to claim 1 wherein said capsule tube andimpervious end plugs are a platinum-rhodium alloy and said porousmembers are pressed and sintered particulate stainless steel forretaining precipitate particles having dimensions in excess of about10-20 microns.
 3. The method according to claim 1 wherein said slurryadditionally contains terbium oxalate.
 4. The method according to claim1 wherein said californium oxalate is calcined to californium oxide.